The present invention relates to optical fibres and in particular to dispersion shifted fibres intended to be used in wavelength division multiplex transmission systems.
Monomode optical fibres referred to as dispersion shifted fibres (DSF) are used at wavelengths around 1 550 nm, and more generally in a window of wavelengths from 1 500 nm to 1 600 nm. Silica has non-zero chromatic dispersion at these wavelengths (in contrast to its chromatic dispersion in the transmission wavelength window around 1 300nm). The chromatic dispersion of the silica is compensated, in particular by increasing the index difference xcex94n between the core of the fibre and its optical cladding, to limit chromatic dispersion of the wave transmitted in the transmission window of dispersion shifted fibres. In practice this index difference shifts the wavelength at which chromatic dispersion is cancelled out; it is obtained by introducing dopants into the fibre during its fabrication, for example by an MCVD process known in the art, which is not described in more detail here. A typical value for the index difference between the cladding and the core of the fibre is 10xc3x9710xe2x88x923 to 14xc3x9710xe2x88x923 ; the index can be increased in the silica by using germanium as the dopant.
Monomode dispersion shifted fibres must also have low curvature losses and low attenuation, just like conventional line fibres.
What is more, using dispersion shifted fibres in wavelength division multiplex transmission systems which transmit RZ, NRZ or soliton pulses is subject to further constraints, all the more so as the number of channels transmitted, the bit rate of each channel and the power on the output side of the amplifier increase and the spacing between channels decreases. Thus it is preferable to use a fibre having sufficiently high chromatic dispersion in the transmission window to prevent the phenomenon of four-wave mixing. Fibres are therefore used which have a chromatic dispersion cancellation wavelength xcex0 other than 1 550 nm to prevent the problems caused by four-wave mixing. These fibres are referred to as non-zero dispersion shifted fibres (NZ-DSF).
Finally, to avoid non-linear effects, the fibres must have a large effective core area, typically greater than 70 xcexcm2.
M. Kato et al., xe2x80x9cA new design for dispersion shifted fibre with an effective core area larger than 100 xcexcm2 and good bending characteristicsxe2x80x9d, ThK1, OFC""98 Technical Digest, explains that non-linear effects in the fibres could become the dominant limitation on transmission capacity and distance for long-haul high-capacity amplified transmission systems. The document specifies that one possible solution is to increase the effective core area of the fibres, which produces a higher power and a greater distance between repeaters. The document proposes a fibre having a profile referred to as a coaxial profile, surrounded by a pedestal, with an effective core area of 146 xcexcm2 and a chromatic dispersion cancellation wavelength xcex0 of 1 500 nm. The chromatic dispersion at 1 550 nm is low and the dispersion slope at this wavelength is 0.09 ps/nm2.km.
EP-A-0 789 255 describes dispersion shifted fibres with high effective core areas, greater than 200 xcexcm2. The fibres have wavelengths xcex0, greater than 1 550 nm. One example of a family of fibres has a value of xcex0 at 1 580 nm, an effective core area of 265 xcexcm2 and a chromatic dispersion slope of 0.085 ps/nm2.km.
The fibres described in the above two prior art documents have the disadvantage of very low chromatic dispersion at 1 550 nm; this value of the chromatic dispersion at 1 550 nm does not prevent four-wave mixing.
The object of the invention is to provide a dispersion shifted fibre with a high effective core area which also has sufficiently high chromatic dispersion at 1 550 nm to prevent four-wave mixing.
To this end the invention proposes a monomode dispersion shifted optical fibre having an effective core area greater than 100 xcexcm2, characterized in that it has a chromatic dispersion cancellation wavelength xcex0 from 1 400 nm to 1 500 nm and low curvature losses.
In one embodiment of the invention, the chromatic dispersion cancellation wavelength xcex0 is from 1 450 nm to 1 500 nm and preferably approximately 1 480 nm.
The fibre advantageously has a chromatic dispersion from 7.5 ps/nm.km to 10 ps/nm.km for a wavelength of 1 550 nm.
In one embodiment of the invention, the fibre has an effective core area from 110 xcexcm2 to 125 xcexcm2.
In another embodiment of the invention, 100 turns of fibre with a radius of 30 mm have an attenuation less than or equal to 0.1 dB at a wavelength of 1 670 nm.
A first embodiment of a fibre according to the invention has an index profile with a core surrounded by an optical cladding, said core including, coaxially and starting from the axis of the fibre:
a trapezium-shaped central part,
an intermediate area in which the index is lower than the maximum index of the central part, and
a ring in which the index is lower than the maximum index of the central part and higher than that of the intermediate area.
The profile advantageously includes between the ring and the cladding a second intermediate area in which the index is lower than that of the cladding.
In a second embodiment of the invention, the fibre has an index profile with a core surrounded by an optical cladding, said core including, coaxially and starting from the axis of the fibre:
a central part in which the index is lower than or substantially equal to that of the cladding,
a peripheral area in which the index is higher than that of the central part,
an intermediate area in which the index is lower than that of the central part, and
a ring in which the index is higher than that of the cladding.